IV — 1. Introduction

The AP600 and AP1000 are pressurized light water reactors designed by the Westinghouse Electric Corporation to produce 600 MW and 1100 MW of electric power, respectively. Both designs employ passive safety systems that rely on gravity, compressed gas, natural circulation, and evaporation to provide for long term cooling in the event of an accident.

Figure V-1 shows the overall layout of the plant. Figure V-2 is a schematic that illustrates the primary system components. The primary loop consists of the reactor vessel, which contains the nuclear fuel assemblies; two hot legs, which connect the reactor vessel to the steam generators; two steam generators; a pressurizer; four canned motor pumps; and four cold legs.

FIG. V-2. Schematic ofprimary loop of the AP600 and AP1000 plants.

During normal power operation, heat is generated in the reactor fuel. This heat is transported by conduction through the fuel and its cladding and transferred by convection to the water. Since the entire system operates at 15.5 MPa (2250 psia), bulk boiling of the water does not occur. The heated water is transported through the hot legs to the U tubes inside the steam generators. The energy of the primary coolant inside the tubes is transferred to the water on the secondary side by forced convection inside the tubes, conduction through the tube walls and boiling on the outside surface of the U tubes. The cooled water leaving the steam generator is pumped by four canned motor pumps, through four cold legs, back into the reactor vessel where the heating cycle is repeated. Primary system pressure is maintained constant by the pressurizer.